The European Telecommunications Standard Institute-Digital Mobile Radio (ETSI-DMR) is a direct digital replacement for analog Private Mobile Radio (PMR). DMR is a scalable system that can be used in unlicensed mode (in a 446.1 to 446.2 MHz band), and in licensed mode, subject to national frequency planning. Any of the ETSI standards or specifications referred to herein may be obtained by contacting ETSI at ETSI Secretariat, 650, route des Lucioles, 06921 Sophia-Antipolis Cedex, FRANCE.
DMR promises improved range, higher data rates, more efficient use of spectrum, and improved battery. Features supported include fast call set-up, calls to groups and individuals, short data and packet data calls. The communications modes include individual calls, group calls, and broadcast calls provided via a direct communication mode among the radios operating within the network. Other important DMR functions such as emergency calls, priority calls, full duplex communications, short data messages and Internet Protocol (IP)-packet data transmissions are supported.
Direct mode is a mode of operation where radios may communicate within a network without the assistance of one or more infrastructure equipment. A radio, as used herein, can be mobile and/or fixed end equipment that is used to obtain DMR services. Direct mode is a communication technique where any radio can communicate with one or more other radios without the need for any additional infrastructure equipment (e.g. base stations or repeaters). Direct mode operation is in contrast to the conventional repeater mode which is a mode of operation where radios communicate through infrastructure equipment such as a repeater. Direct mode, therefore, can provide a more efficient, less costly communication system operation than repeater mode operation.
The European Telecommunications Standard Institute-Digital Mobile Radio (ETSI-DMR) standard provides for 6.25e (2:1 TDMA) operation in repeater mode. 6.25e (2:1 TDMA) operation refers to 6.25 Kilohertz (kHz) equivalent spectral efficiency. As there is no restriction on what happens in neither each time slot nor any interrelation between them (other than the need to maintain time synchronicity), it is therefore possible to have two entirely separate conversations at the same time from two different units. By this means it is possible that two simplex calls can be independently supported in a single 12.5 kHz channel. Secondly, this means that DMR devices fitted with this protocol will also comply with the North American requirements for 6.25 kHz channel equivalence.
In contrast, the present ETSI-DMR standard only provides for 12.5 Kilohertz (KHz) operation in talkaround mode (systems that primarily use a repeater and occasionally communicate without a repeater) and direct mode (systems that exclusively communicate without a repeater). 12.5 KHz operation refers to 12.5 KHz spectral efficiency in which there is only one communication path per 12.5 KHz of radio frequency (RF) spectrum.
Both 6.25e repeater mode and 12.5 talkaround/direct mode utilize 27.5 millisecond (msec) pulsed (every 60 msec) radio transmissions. In the 6.25e repeater mode of operation, the repeater defines the TDMA time slot boundaries on the channel and the radios synchronize themselves to the repeater for transmitting and receiving. The radios' transmissions are pulsed. The repeaters transmit a continuous signal divided into time slots. In the 12.5 talkaround/direct mode of operation, radios transmit asynchronously and radios within range of the transmission synchronize themselves to that transmission for the purposes of receiving the transmission, but any transmissions in response to the first transmission are transmitted asynchronously.
Accordingly, there is a need to support more spectrally efficient direct mode, specifically, 2:1 TDMA or 6.25e direct mode. To support this, all radios need to be synchronized to use a common channel timeslot structure. Therefore, there is a need for synchronizing direct mode TDMA transmissions in a wireless communication system.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.